The present invention relates to semiconductor laser control circuits for use in large-capacity optical communication systems, and more particularly to a laser control circuit in which temperature compensation is achieved by controlling the bias current.
In a prior art laser driving circuit for high-speed digital optical communication systems, a bias current I.sub.b, slightly below a current I.sub.th (hereinafter referred to as the threshold current) at which the lasing action starts, is constantly fed to the semiconductor laser, and a pulse current corresponding to a digital modulating signal is caused to overlap the bias current I.sub.b to obtain an optical pulse signal. As the threshold current of the semiconductor laser varies with temperature, the bias current I.sub.b is so controlled as to keep the optical output power of the semiconductor constant. A semiconductor laser control circuit in which the bias current is controlled in this manner is disclosed in the U.S. Pat. No. Re. 31,969 (Shell). In Sell's laser control circuit, part of the laser's output light is detected with a light receiving element. The difference between a voltage corresponding to output level of the light receiving element. and another voltage proportional to the average voltage of an electrical modulating signal is amplified. Next, the amplified output is supplied to the laser as a prebias signal, thereby keeping the optical output power constant without relying on temperature and the pulse density of the electrical modulating signal.
However, at, higher temperatues, the bias current has to be increased above the threshold current I.sub.th to keep the optical output constant. This is necessary since, curve representing the relationship between the current and the optical output power (hereinafter referred to as the differential quantum efficiency, abbreviated to DQE) varies with temperature T, and in particular, with declines high temperature. Thus, a ratio (I.sub.b /I.sub.th) between the bias current I.sub.b and the threshold current I.sub.th increases with high temperature. Generally in a semiconductor laser, the higher the I.sub.b /I.sub.th ratio, the greater the intersymbol interference, combined with an increase in the speed of modulating signals.